A voltage sag, or voltage dip is a reduction of the voltage at a customer position with a duration of between one cycle and a few seconds. Voltage sags are caused by motor starting, short circuits and fast re-closing of circuit breakers. Voltage sags normally do not cause equipment damage but can easily disrupt the operation of sensitive loads such as electronic adjustable speed drives (ASDs). A severe voltage sag can be defined as one that falls below 85% of rated voltage. Power quality surveys are a common practice and frequently appear in the literature. According to these surveys, voltage sags are the main cause of disturbances. For example, in one survey, 68% of the disturbances registered were voltage sags, and these sags were the only cause of production loss. This loss was caused by voltage drops of more than 13% of rated voltage and a duration of more than 8.3 ms (1/2 cycle). A recent study conducted at two industrial sites with adjustable speed drives concluded that voltage sags with a duration of 12 cycles or more and lower than 20% voltage drop will trip out the adjustable speed drive (either over-current or under-voltage trip) involved in a continuous process. Modern adjustable speed drives appear to be more sensitive than data processing equipment to voltage sags.
For example, in textile and paper mills a brief voltage sag may potentially cause an adjustable speed drive to introduce speed fluctuations that can damage the end product. Further, a brief voltage sag also causes a momentary decrease in dc-link voltage triggering an under voltage trip or result in an over current trip. Such nuisance tripping of adjustable speed drive equipment employed in continuous-process industries contributes to loss in revenue and can incur other costs.
Currently, there are boost system which are designed to maintain the dc-link voltage under voltage sags. These methods however, introduce diodes that are in the series path of power flow They also include an inductor that is bulky and is in the series path of power flow. Having the diodes and inductor in the series path of the main power flow is disadvantageous because it causes power to dissipate and can become a reliability problem if those components were to fail, since they are in series with the main power flow. Also, the inductor has to be large because it carries power at all times. Additionally, the inductor in this design carries high frequency current during the boost mode when voltage sags occur.
Other methods to provide ride-through including adding motor generator sets, adding flywheel energy storage and using super conductor magnetic energy storage (SMES). These methods are prohibitively expensive.